An arrhythmia is a disturbance in the normal rate, rhythm or conduction of the heartbeat. A tachyarrhythmia is a condition in which an arrhythmia is occurring at a high rate, e.g., 100 or more beats per minute. Arrhythmias may be classified broadly by the chamber of the heart in which the arrhythmia originates. Such classification includes supraventricular tachycardia, ventricular tachycardia, or concomitant supraventricular and ventricular tachycardia, defined here as “double tachycardia.” Tachycardias can lead to fibrillation, which is characterized by a chaotic activation of myocardial tissue. Ventricular tachycardia and ventricular fibrillation can be life threatening over a period of minutes.
Treatment for tachyarrhythmias may include anti-tachycardia pacing (ATP), cardioversion, or defibrillation in which a train of high rate pulses or one or more high-energy pulses is delivered to the heart in an attempt to restore a more normal rhythm. ATP is typically effective in converting stable tachyarrhythmias to normal rhythm, and is often delivered via an implanted device. In some cases, a sequence of increasingly aggressive ATP therapies are applied until an episode of tachyarrhythmia is terminated. Some implanted devices can be configured to discontinue ATP and immediately apply cardioversion in the event the tachyarrhythmia degrades into fibrillation.